Dislocations are commonly present and important in metals but their effects have not been fully recognized in oxide ceramics. The large strain energy raised by the rigid ionic/covalent bonding in oxide ceramics leads to dislocations with low density (∼106 mm−2), thermodynamic instability and spatial inhomogeneity. In this paper, we report ultrahigh density (∼109 mm−2) of edge dislocations that are uniformly distributed in oxide ceramics with large compositional complexity. We demonstrate the dislocations are progressively and thermodynamically stabilized with increasing complexity of the composition, in which the entropy gain can compensate the strain energy of dislocations. We also find cracks are deflected and bridged with ∼70% enhancement of fracture toughness in the pyrochlore ceramics with multiple valence cations, due to the interaction with enlarged strain field around the immobile dislocations. This research provides a controllable approach to establish ultra-dense dislocations in oxide ceramics, which may open up another dimension to tune their properties.

A 2 B 2 O 7 and A 3 BO 7 transparent ceramic families are potential materials for optical lenses because of their high refractive index. Although nonstoichiometry is widely present in these material families, its effect on refractive index and optical propertieshas not yet been fully studied. In this study, optical properties are reported for the Y 3 NbO 7 transparent ceramic series, Y 1−x Nb x O 1.5+x (x = 0.20, 0.22, 0.24, 0.25, 0.26), which were fabricated by a pressureless pre-sintering and a hot isostatic pressing postsintering treatment. The refractive index increases from 2.04 to 2.10 (at 587.6 nm) as the Nb content x increases, which is mainly attributed to the variation in the oxygen ion/vacancy ratio. The Abbe number is larger than 40, showing a decreasing trend as the Nb content x increases. The specimen with x = 0.24 has the highest inline transmittance, which were 62% and 76% at 587.6 and 2000 nm, respectively, for a 1-mm-thick specimen. Through the approach of nonstoichiometry, Y 1−x Nb x O 1.5+x series exhibit balanced properties of refractive index, Abbe number, and transmittance, which can be considered as a promising candidate for high refractive index optical lenses. K E Y W O R D Sdefect fluorite, optical materials/properties, refractive index, transparent ceramics SUPPORTING INFORMATIONAdditional supporting information may be found online in the Supporting Information section.